Changes of PI - implementation of substantial amendment.

I work on behalf of a sponsor of a study with a large number of sites. I am getting conflicting opinions about the process for change of PI at a site. Most sites appear happy to notify us of a change, and at the same time advise their R&D department about the new PI. We have been working on the basis that the change of PI can be implemented from the point that the R&D department indicates they are happy with the change, albeit informally (e.g. email confirmation). We then submit a substantial amendment to REC in batches once we have a few changes to process. We forward the final opinion once available back to the sites and get the formal confirmation of extension of NHS permission at that point. However, recently I have been told that this is not appropriate, and as it is a substantial amendment, the change of PI cannot be implemented until the whole process has been completed. The assumption is that trial activities and recruitment of new patients must cease until this point.

I would welcome people's views, and how other studies manage this. I am particularly talking about cases where the proposed new PI is in place - not circumstances where there is a gap in cover. Can there be an assumption of the proposed PI taking an 'acting PI role' and continuation of all trial activities on this basis?

As we have such a large number of sites, we do PI changes substantial amendments in batches hence there can be a delay in submitting (and receiving REC opinion back). What is the MHRA's view of this?